1. Technical Field of the Invention
The invention relates to a process for preparing a precursor solution for polyimide/silica composite material, a process for forming a polyimide/silica composite material film on a substrate, the resulting precursor solution and composite material, and the use thereof.
2. Prior Art
Metallic, ceramic, polymeric and electronic materials are currently four primary areas of materials science. Each type of the materials has its special properties, merits and faults. For example, polymeric materials are processable, flexible, elastic, corrosion resistant, insulating and cheap, but they have relatively poor heat resistance and mechanical strength. Ceramic materials are rigid and less active with excellent heat resistance and mechanical strength, but they are heavier and friable. Brand new materials with excellent properties may be made through combining the advantages of various materials while remedying their shortcomings. Under this notion, widespread researches have been carried out on organic-inorganic hybrid materials, i.e. composite materials.
The domain of conventional composite materials usually ranges from hundreds of microns to centimeter grade. Organic or inorganic components of such materials mainly play a role of the changing the structures or functions of the materials, and the materials are normally prepared by physically blending these components. Hybrid materials, however, are normally prepared through chemical methods, such as sol-gel or self-assembly methods, which remedy the shortcomings of the composite materials by the microscopic mixing of the organic and inorganic components. For example, the friable property of inorganic materials may be improved and various colors are available when an organic material is introduced into the matrix of an inorganic material. Alternatively, when an inorganic material is introduced into the matrix of an organic material, the mechanical strength and heat resistance can be increased and the hygroscopic property will be improved.
Generally, common organic-inorganic hybrid materials have to be heated to a relatively high temperature so as to remove the solvent in the system and to accomplish the required crosslinking reaction of the inorganic components with the removal of moisture. Polyimide has been widely used in semiconductor and printed circuit board industries due to its better mechanical property and heat resistance over conventional polymeric materials. Accordingly, polyimide/silica composite materials have drawn large attention, and extensive researches have been carrying out on such materials to obtain better properties and improve the shortcomings thereof.
At present, a polyimide/silica composite material is normally prepared by the following methods:    (1) A dianhydride and a diamine are added to a common solvent such as dimethylacetamide (DMAc) or N-methylpyrrolidine (NMP) to react with each other to produce poly(amic acid) (PAA). Water and a catalyst (either acidic or basic catalyst) are added to tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) in ethanol or tetrahydrofuran (THF), which is then hydrolyzed and polycondensed to produce oligomeric silica particles or micelles, and then the PAA solution is homogeneously stirred with the silica solution to obtain a precursor solution for polyimide/silica composite materials.    (2) A dianhydride and a diamine are added to a common solvent such as DMAc or NMP to react with each other to produce PAA, to which a monomer of TEOS or TMOS is directly added. The hydrolysis and polycondensation reaction of silica is directly carried out by utilizing the PAA as a catalyst to obtain a precursor solution for polyimide/silica composite material.    (3) A dianhydride is reacted with a diamine to produce PAA. An amino coupling agent such as 3-aminopropyltetraethoxysilane (APrTEOS) is added to the acid anhydride end of the PAA. Thereafter, TEOS or TMOS is added and the hydrolysis and polycondensation reaction occurs, such that a covalent bonding between the polyimide in the organic phase and the silica in the inorganic phase is created through the coupling agent, which enhances the compatibility between the organic phase and the inorganic phase so as to reduce the size of the silica particles and improve the distribution uniformity thereof to achieve better properties.    (4) A dianhydride is reacted with a diamine to produce PAA. Thereafter, TEOS or TMOS is directly added to the mixture. The hydrolysis and polycondensation reaction of silica is directly carried out by utilizing PAA as a catalyst. In addition, a coupling agent such as γ-glycidyloxypropyltrimethoxysilane (GTMOS) is added to enhance the compatibility between the organic phase and the inorganic phase through the intermolecular force such as hydrogen bonds to reduce the size of the silica particles and improve the distribution uniformity thereof to achieve better properties.    (5) A dianhydride is reacted with a diamine to produce PAA. An amino coupling agent such as 3-aminopropyltetraethoxysilane (APrTEOS) is added to the acid anhydride end of the PAA. Thereafter, TEOS or TMOS is added and the hydrolysis and polycondensation reaction occurs. In addition, a coupling agent such as γ-glycidyloxypropyltrimethoxysilane (GTMOS) is added to enhance the compatibility between the organic phase and the inorganic phase through intramolecular covalent bonds (provided by APrTEOS coupling agent) and the intermolecular force such as hydrogen bonds (provided by GTMOS coupling agent), so as to reduce the size of the silica particles and improve the distribution uniformity of the silica particles. High performance composite materials having better properties over the original polyimide may be manufactured through the above mentioned conventional methods for preparing polyimide/silica composite materials. However, when such a material is utilized to produce microstructures or specific, functional patterns on wafers or glass substrates, since the material is non-photosensitive, use must be made of a conventional lithography process to make the patterns after the film is cured, including the coating of a photoresist, exposing and developing of the photoresist, etching of the composite material film by means of reactive ion etching and the like, and stripping and cleaning of the residual photoresist with ozone and specific chemicals. The steps of the process are complicated and time-consuming. In addition, the parameter design during the etching of the polyimide/silica composite material is difficult due to its excellent resistance to etching. Moreover, the patterns after the etching process are susceptible to incomplete etching and high side wall roughness, and residual photoresist thereon.